Hutton et al., U.S. Pat. No. 4,033,059, shows a method of printing documents including transitory images. The transitory images of that patent are printed in ink lines which contrast in color with the underlying paper or other substrate. The images in certain embodiments of that invention are invisible when viewed perpendicularly to the plane of the paper, but appear when viewed at a small angle to the plane of the paper. Such images are termed "latent images". In other embodiments, the images are visible when viewed along a line of sight perpendicular to the plane of the paper, but disappear when viewed at a small angle. In that case, the images are termed "transient images". The term "transitory image" is defined in the Hutton et al. patent as a generic term inclusive of both latent and transient images.
The U.S. Pat. No. 4,124,947 to Kuhl et al., shows a modification of the documents of Hutton et al., in which the printed intaglio lines have spaced aligned transverse "passageways or streets" having a small width as compared to the spacing between the intaglio lines.
The U.S. Pat. No. 3,471,172, to Bayha, suggests a document in which intaglio lines are printed in ink of a color which has no contrast with respect to the substrate on which the lines are printed. These lines are asserted by Bayha to be invisible and only detectable by an apparatus which detects infrared radiation transmitted through the printed and unprinted areas of the substrate. The documents proposed by Bayha present extreme difficulty in printing, because the press operator cannot tell by inspecting the printed documents when the press is adjusted to supply a proper quantity of ink to the documents. The operator must have an infrared detection apparatus to enable him to adjust the press properly.